Thermodynamic properties of copper carbonates ? malachite Cu2(OH)2CO3 and azurite Cu3(OH)2(CO3)2

Abstract

The thermodynamic properties of the copper carbonates malachite and azurite have been studied by adiabatic calorimetry, by heat-flux Calvet Calorimetry, by differential thermal analysis (DTA) and by thermogravimetrie (TGA) analysis. The heat capacities, Cp0of natural malachite and azurite have been measured between 3.8 and 300 K by low-temperature adiabatic calorimetry. The heat capacity of azurite exhibits anomalous behavior at low temperatures. At 298.15 K the molar heat capacities Cp0and the third law entropies S298.150are 228.5±1.4 and 254.4±3.8 J mol−1 K−1 for azurite and 154.3±0.93 and 166.3±2.5 J mol−1 K−1 for malachite. Enthalpies of solution at 973 K in lead borate 2PbO·B2O3 have been measured for heat treated malachite and azurite. The enthalpies of decomposition are 105.1±5.8 for azurite and 66.1±5.0 kJ mol− for malachite. The enthalpies of formation from oxides of azurite and malachite determined by oxide melt solution calorimetry, are −84.7±7.4 and −52.5±5.9 kJ mol−1, respectively. On the basis of the thermodynamic data obtained, phase relations of azurite and malachite in the system Cu2+-H2O-CO2 at 25 and 75 °C have been studied.